1. Field of the Invention
This invention relates to a photographic camera, and more particularly to a photographic camera having a density-variable optical filter for controlling exposure. The photographic camera in accordance with this invention is capable of controlling the exposure by varying the density of the optical filter as well as by changing the shutter speed or the aperture size of a diaphragm.
2. Description of the Prior Art
In the conventional photographic camera, either the aperture size of a diaphragm or the shutter speed is freely selected in advance and the remaining adjustment is made manually or automatically to take a picture of normal exposure. In addition to this ordinary type of photographic camera, it has also been known in the art to select a combination of the aperture size and the shutter speed from a group of predetermined combinations thereof in accordance with the scene brightness. In this type of camera, the combination is usually automatically selected from the group.
In these conventional photographic cameras, only two variables, i.e., the aperture size and the shutter speed, are used to control the exposure. The film sensitivity is of course introduced into the consideration of the exposure control in advance. Therefore, if one of the two variables is selected in advance, the other is automatically determined in accordance with the scene brightness to obtain a photograph of normal exposure.
However, in the photographic technique, the aperture size has an important role in addition to that of exposure control, namely, that of determining the focal depth. Further, the shutter speed has also an important role in determining the degree of blur. Therefore, a large aperture size or a slow shutter speed is sometimes employed intentionally to obtain a medium sharp or a somewhat blurred photograph. Thus, it is desirable to be able to freely select both the aperture size and the shutter speed to obtain a photograph of desirable sharpness and blur.
It has been known in the art to use an optical filter in a photographic camera in which either the aperture size of a diaphragm or the shutter speed is freely selected and the remaining one of these is controlled to obtain a picture of normal exposure. Such an optical filter has been used for the purpose of correcting over-exposure when the scene brightness is too high. For instance, when the picture is taken at a place where the ultra-violet rays are strong such as near the sea or in the mountain, an ultra-violet ray absorbing filter or a neutral density filter is employed. These filters are only used for the purpose of correcting over-exposure, and accordingly, it is impossible to freely select the aperture size and the shutter speed even if the filter is used.
Further, it has also been known in the art to provide a density-variable filter in a camera for the purpose of obtaininglarge F-numbers such as 32, 45, 90 and so on over the limit of the aperture size. One example of such a camera with a density-variable optical filter is disclosed in U.S. Pat. No. 3,732,793, in which a polarization filter ring graduated with said large F-numbers is rotatably mounted on a camera to attenuate the light incident upon the film therein by rotation thereof in combination with another polarization filter mounted on the camera. This density-variable filter, however, is regarded as a part of the diaphragm to enlarge the range of the aperture size thereof. Therefore, in this camera with a variable-density filter also, it is impossible to freely select both the shutter speed and the aperture size as desired. This camera is also, therefore, classified as a two-variable type camera.